The present invention relates to an ink jet recording device, in particular, to a method for driving an ink jet recording head that ejects minute ink droplets from nozzles and prints characters and images, and to an ink jet recording device.
Concerning an ink jet recording device that ejects minute ink droplets from nozzles and prints characters and images, for example, as disclosed in Japanese Patent Application Laid-Open No. SHO53-12138 and Japanese Patent Application Laid-Open No. HEI10-193587, a drop-on-demand type ink jet is well known, in which a pressure wave (acoustic wave) is generated in a pressure generating chamber filled with ink by using a driving device such as a piezoelectric actuator that converts electric energy into mechanical energy such as vibration, and an ink droplet is ejected from a nozzle connected to the pressure generating chamber.
FIG. 13 is a diagram showing an example of a recording head in an ink jet recording device well known by the above described patent applications, etc. A nozzle 62 for ejecting ink and an ink supply channel 64 for leading ink from an ink tank (not shown) through a common ink chamber 63 are connected to a pressure generating chamber 61. Further, a diaphragm 65 is set at the bottom of the pressure generating chamber.
When ejecting ink droplets, the diaphragm 65 is displaced by a piezoelectric actuator 66 set to the outside of the pressure generating chamber 61, and volume in the pressure generating chamber 61 is changed. Thereby, a pressure wave is generated in the pressure generating chamber 61. By the pressure wave, a part of the ink which fills the inside of the pressure generating chamber 61 is ejected outward through the nozzle 62 as an ink droplet 67. The ejected ink droplet reaches the surface of a recording medium such as recording paper, and forms a recording dot. By repeating the formation of the recording dot based on image data, characters and images are recorded on the recording paper.
In order to acquire high image quality using this kind of ink jet recording head, it is necessary to make the diameter of an ejected ink droplet very small. Namely, in order to obtain a smooth image with low granularity, it is necessary to make the recording dot (pixel) formed on the recording paper as small as possible. For that purpose, the diameter of the ejected ink droplet has to be set smaller.
Generally, when the dot diameter becomes 40 xcexcm or less, the granularity of the image decreases to a large extent. Further, when it becomes 30 xcexcm or less, it becomes difficult to visually recognize each dot even at a highlight section of the image, and thereby, the image quality can be drastically improved. The relationship between the ink droplet diameter and the dot diameter depends on the flying speed of the ink droplet (droplet velocity), the physical property of the ink (e.g. viscosity and surface tension), the kind of the recording paper, etc. Nevertheless, the dot diameter generally becomes approximately twice as large as the ink droplet diameter. Therefore, in order to obtain the dot diameter not exceeding 30 xcexcm, it is necessary to set the ink droplet diameter to 15 xcexcm or less.
Incidentally, in this specification, the drop diameter is defined as the diameter of one spherical ink droplet in the same amount as the total amount of the ink (including satellites) ejected at a time.
The most effective means of reducing the ink droplet diameter includes a reduction of a nozzle diameter.
However, because of the limit of manufacturing technology, and problems in reliability such as clogging of a nozzle, etc., the lower limit of the nozzle diameter is 20 to 25 xcexcm for actual use, and thereby, it is difficult to obtain an 15 xcexcm level ink droplet only by the reduction of the nozzle diameter. Consequently, there have been made some attempts to reduce the ejecting ink droplet diameter by driving methods, and some efficient methods have been proposed.
As a driving method for realizing the ejection of a minute droplet with an ink jet recording head, there is known a driving method in which a pressure generating chamber is once inflated just before ejection, and the ejection is conducted from the state where a meniscus at a nozzle opening section is pulled toward the side of the pressure generating chamber (for example, Japanese Patent Application Laid-Open No. SHO55-17589).
An example of a driving waveform used in this kind of driving method is shown in FIG. 14.
While the relationship between a driving voltage and operation of a piezoelectric actuator varies according to the configuration and the polarized direction of the actuator, it is assumed in this specification that, when the driving voltage is increased, the volume of the pressure generating chamber is reduced, and contrary, when the driving voltage is reduced, the volume of the pressure generating chamber is increased.
The driving waveform shown in FIG. 14 comprises a voltage changing section 141 for inflating the pressure generating chamber and a voltage changing section 142 for subsequently compressing the pressure generating chamber and ejecting ink droplets.
FIGS. 15(a) to 15(d) are pattern diagrams showing the movement of the meniscus at the nozzle opening section when applying the driving waveform shown in FIG. 14.
In an initial state, the meniscus is formed of a flat shape (FIG. 15(a)). When the pressure generating chamber 61 is expanded just before the ejection, the central part of the meniscus is pulled toward the pressure generating chamber 61, and thereby, the shape of the meniscus becomes concave as shown in FIG. 15(b).
From this state, when the pressure generating chamber 61 is compressed by the voltage changing section 142, the central part of the meniscus is pushed out of the nozzle 41, and a thin liquid column 43 is formed as shown in FIG. 15(c). Subsequently, as shown in FIG. 15(d), the tip of the liquid column 43 is separated, and an ink droplet 44 is formed.
The droplet diameter of the ink droplet 44 is approximately the same as that of the formed liquid column 43, and is smaller than that of the nozzle 41. Namely, by using that kind of driving method, it is possible to eject ink droplets smaller than the nozzle in diameter.
Incidentally, as described above, the driving method in which minute droplet ejection is conducted by controlling the meniscus shape just before the ejection will be hereinafter referred to as a xe2x80x9cmeniscus control methodxe2x80x9d in this specification.
As described above, by using the meniscus control method, it becomes possible to eject ink droplets smaller than the nozzle in diameter. However, when using the driving waveform as shown in FIG. 14, approximately 25 xcexcm is the smallest limit to the droplet diameter obtained in actuality, which cannot be enough to meet recent increasing needs for higher image quality.
Consequently, the present inventor proposed, in Japanese Patent Application Laid-Open No. HEI10-318443, a driving waveform as shown in FIG. 16 as a driving method for enabling further minute droplets to be ejected. This driving waveform comprises a voltage changing section 151 for pulling in the meniscus just before the ejection, a voltage changing section 152 for compressing the pressure generating chamber and forming the liquid column, a voltage changing section 153 for early separating the droplet from the tip of the liquid column, and a voltage changing section 154 for controlling reverberation of the pressure wave remaining after the ink droplet ejection.
Namely, the driving waveform of FIG. 16 is such that pressure wave control aiming at the early separation of the ink droplet and the reverberation control is added to the conventional meniscus control method, and thereby, it becomes possible to eject an ink droplet having an approximately 20 xcexcm droplet diameter stably.
In addition, the present inventor developed an ejection method utilizing natural vibration of a piezoelectric actuator as a method for ejecting minute droplets each having a droplet diameter of 15 xcexcm or less, and disclosed a driving waveform as shown in FIG. 17 in Japanese Patent Application Laid-Open No. HEI11-20613.
This driving waveform also comprises, as with the driving waveform of FIG. 16, a voltage changing section 161 for pulling in the meniscus just before the ejection, a voltage changing section 162 for compressing the pressure generating chamber and forming the liquid column, a voltage changing section 163 for early separating the droplet from the tip of the liquid column, and a voltage changing section 164 for controlling reverberation of the pressure wave remaining after the ink droplet ejection.
This driving waveform is characterized by setting a voltage changing period t3 of the second voltage changing process and a voltage changing period t5 of the third voltage changing process equal to or less than resonance frequency Ta of the piezoelectric actuator itself. Thus the natural vibration of the piezoelectric actuator itself is excited, and vibration having high frequency is generated in the meniscus. By combining such setting with the above described meniscus control method, droplets smaller than those achieved by the general meniscus control method can be ejected.
However, when using the above described driving method utilizing the natural vibration of the piezoelectric actuator in order to acquire smaller ink droplets, the deformation speed of the piezoelectric actuator is increased, and thereby, a problem with ensuring the reliability of the piezoelectric actuator arises.
Further, in order to excite the natural vibration of the piezoelectric actuator, it is necessary to apply a driving waveform having very short rising/falling time to the piezoelectric actuator. Thereby, the current passing through the driving circuit of the piezoelectric actuator increases. As the current passing through the driving circuit increases, the heating value from the circuit also increases as well as cost for the circuit components such as switching IC is driven up, and thereby, countermeasures for heat release is required, which cause the increase in the cost of the driving circuit system and the size of the device.
For the above reasons, the driving method utilizing the natural vibration of the piezoelectric actuator has not been put to practical use yet, and the ejection of minute droplets of 20 xcexcm or less with a low-cost device has been extremely difficult in practice.
Further, another problem in conducting the ejection of the minute droplets each having a droplet diameter of 20 xcexcm or less is an ejection characteristic change caused by production variations. Namely, while the ink jet recording heads are manufactured by micro-fabrication technology and precise assembly technology, the ejection characteristics of the heads are subtly changed because of the variations of the component sizes and manufacturing conditions.
Concretely, changes occur to the resonance frequency and the amplitude of the pressure wave generated in the pressure generating chamber. As described above, the minute droplet ejection by the meniscus control method is a technique in which the ink in the nozzle is controlled with high accuracy. Thereby, the ejection is very sensitive to the changes of the ejection characteristics, and the tolerance of the ejection characteristic becomes very narrow. Therefore, there have been problems that the yield at manufacturing the heads gets worse, and that the manufacturing cost increases to a large extent.
The present invention has been made so as to overcome the above problems, and accordingly, the object of the present invention is to provide a driving method of an ink jet recording head and the device thereof, which enables ejection of minute droplets each having a diameter of 20 xcexcm or less without increasing the device cost and size and decreasing the reliability.
According to the present invention, there is provided a driving method of an ink jet recording head to realize the above objects, for ejecting an ink droplet from a nozzle connected to the pressure generating chamber by applying driving voltage to a driving device, driving the driving device and generating a pressure change in a pressure generating chamber filled with ink, wherein:
a voltage waveform of the driving voltage at least comprises a first voltage changing process for inflating the volume of the pressure generating chamber and a second voltage changing process for subsequently deflating the volume of the pressure generating chamber; and
a voltage changing time t1 of the first voltage changing process and a time interval t2 between the finish time of the first voltage changing process and the start time of the second voltage changing process are set so as to satisfy the following relational expression:
t2=t0xe2x88x92t1 
                              t          2                =                              t            0                    -                      t            1                                              (        1        )                                          t          0                =                                            T              c                                      2              ⁢                              xe2x80x83                            ⁢              π                                ⁢                                                    tan                                  -                  1                                            ⁡                              [                                                      sin                    ⁡                                          (                                                                                                    2                            ⁢                                                          xe2x80x83                                                        ⁢                            π                                                                                T                            c                                                                          ·                                                  t                          1                                                                    )                                                                                                  cos                      ⁡                                              (                                                                                                            2                              ⁢                                                              xe2x80x83                                                            ⁢                              π                                                                                      T                              c                                                                                ·                                                      t                            1                                                                          )                                                              -                    1                                                  ]                                      ⁢                          xe2x80x83                        .                                              xe2x80x83            
Incidentally, Tc is pressure wave resonance frequency in the pressure generating chamber.
The invention set forth in claim 2 is the method in which the relational expression of t2 in claim 1 is substituted with:
t0xe2x88x92t1xe2x88x921 xcexcsxe2x89xa6t2xe2x89xa6t0xe2x88x92t1+3 xcexcsxe2x80x83xe2x80x83(2). 
The purpose of the present invention can be realized by the invention claimed in any one of claims 3 to 12.
Namely, conventionally, the mechanism of the minute droplet ejection by the meniscus control was not totally clarified, and further, the driving waveform was not adequately optimized. By contrast, the present inventor has found that, on the basis of a multitude of ejection observing experiments, the minute droplet ejection becomes insensitive to the variations of the pressure wave resonance frequency and also a minute droplet of 20 xcexcm or less can be ejected by setting specific conditions between the voltage changing time t1 of the first voltage changing process and the time interval t2 from the finish time of the first voltage changing process to the start time of the second voltage changing process.
Thereby, it became possible to eject a minute droplet of 20 xcexcm or less without increasing the device cost and size, and decreasing the device reliability and the production yield.